Putting aside the publicly available and free options for things like auto-fill and auto-complete (Off the top of my head is LingPipe, Lucene and Sphinx though) there would be an issue if the server was made to handle lookup and autocomplete every time a client tries to send a tell using this feature.

BUT. There would be less strain if the client side app just tracked the people you whispered recently and do autocomplete on its end. Something like an auto-complete/name suggestion for people you have sent a tell previously would be an ideal candidate for the client side app to handle. All you would need to do is successfully send or receive a tell and then the next time you type /t na.... it will suggest /t name lastname